1. Field of the Invention
The present invention relates to a hologram and more particularly a volume type phase hologram, and further to a method of producing same.
Holography is a unique technique of forming an optical image by which an object is irradiated by a well coherent wave such as laser where the wave is modulated in amplitude and phase in accordance with the shape of the object, the modulated wave reflected upon or transmitted through the object is recorded (=hologram) and the hologram is irradiated again by the laser so as to reproduce an optical image of the original object.
With the recent development of study on holography, it has been made clear to a certain extent what material is suitable for use in holography or what characteristics such a hologram recording material should have.
Thus, there have already been proposed various materials such as bleached silver salt (U.S. Pat. No. 3,672,744), dichromate gelatine (U.S. Pat. No. 3,617,274), photoresist, thermoplastic resin, inorganic glass materials and ferroelectric substances, and at present a further study on the characteristics of these materials is proceeding in the art.
2. Description of the Prior Art
The characteristics which a hologram recording material should have may be summarized as follows:
(1) to have a sensitivity to a laser, especially to a laser in the range of visible wavelength and to have a high sensitivity at the same time;
(2) to have a high resolving power;
(3) to give a hologram of high diffraction efficiency;
(4) to give a hologram of low noise level;
(5) to give a stable hologram; and
(6) to allow an easy recording and reproducing operation.
As will be seen from the above, the requirements for a hologram recording material are very severe ones.
In view of practical purposes, very few known hologram recording materials can satisfy the above requirements completely or at least partially to the extent that its use may be practical.
Among the above mentioned materials, bleached silver salt and dichromate gelatine may be considered to be practically usable. However, they have particular disadvantages. The former necessitates a bleaching treatment in addition to ordinary treatments and furthermore the hologram obtained from it is poor in light fastness. The latter has a difficulty regarding the preservation of hologram because the hologram obtained from this material lacks adequate stability against moisture.